Dewatering process, procedure and device

ABSTRACT

Use of an application of foam to air permeable sheet material by a variety of mechanical or pressure applied means in order to cause or allow the foam to enter the interstices of the material. The foam contains as an essential integer an agent capable of lowering the surface tension of the foaming liquid thereby effecting a dewatering/drying action on the material greater than that than would otherwise be applied.

DESCRIPTION

This invention relates to a foam treatment process for sheet materialsand has particular reference to a process for reducing the water contentof such sheet material.

Ways to reduce the water content of sheet material such as textile sheetmaterial, are well known. The most widely used and oldest known methodinvolves squeezing the sheet material between a pair or several pairs ofmangle rollers. While certain constructions of mangles enable the watercontent to be reduced to low levels (e.g. 40 to 60% depending on thematerial to be treated), mangle-type equipment has severaldisadvantages. The higher the nip pressure the better are the manglingeffects, but, of course, the deformation of the substrate by the nippressure becomes more pronounced.

Another drawback of the mangle principle is the lack of a simple, easilypredictable correlation between nip pressure and the extraction effect.Using water content measuring instrument feedback to control andpredetermine water retention levels is thus very difficult.

Another method frequently used is the vacuum extraction of water fromtextile sheet material. While it is possible to remove a certain amountof the water present in the interstices of the material, the frictionbetween the vacuum slot and the moving sheet presents problems,particularly at high speeds, since adequate sealing become verydifficult. Energy input thus may be too high in relation to the effectsobtained (this is particularly true for all high speed operations).

Another method recommended for the removal of water from air permeablesubstrates is the blowing of air at very high air speeds against thesurface of the moving sheet, usually at an angle of about 90° to theplane of the sheet. Energy input again is very substantial, and resultsvary greatly with the construction of the substrate (tightly woven/openweaves/nonwovens, etc.) while support of the sheet at a low level offriction may present serious problems, particularly in the case of webshaving a low cohesive strength.

All these known treatments which precede the final drying step are aimedat reducing the level of residual water prior to drying to lower theenergy input required to remove the water still present at a given dryerspeed, and/or to increase the speed of the dryer and/or lower the dryingtemperature.

U.S. Pat. No. 4,062,721 describes and claims a method for removing waterfrom a wet fibrous sheet comprising the steps of mixing an aqueousslurry comprising mineral and binder, depositing said aqueous slurry ona wire mesh to form a wet sheet, adding a surfactant foaming agent tothe slurry, said step of adding said surfactant foaming agent beingperformed at substantially the time that said slurry is deposited onsaid wire mesh whereby essentially no internal foam is present in saidwet sheet at the time of depositing, draining water from said wet sheetthrough said wire mesh, said drainage being aided by the force ofgravity and draining additional water from said wet sheet through saidwire mesh, said additional drainage being aided by air pressuredifferential created across the wet sheet whereby foam is generatedwithin the wet sheet due to the passage of air therethrough.

This specification is concerned the production of fire retardant feltedmineral fibre panels and it is a feature of the invention that thegeneration of a foam should be confined to within the felted materialitself. U.S. Pat. No. 4,062,721 teaches with considerable emphasis, theimportance of avoiding substantial foaming until the wet sheet isjuxtaposed the air pressure differential created across the sheet.

We have found that if an air permeable sheet material is treated with afoam containing an agent capable of reducing the surface tension of thefoamed liquid, then improved permeation by the air/liquifying of the airpermeable sheet material, can be effected.

According to the present invention, therefore, there is provided

a process for treating an air permeable sheet material for which processcomprises:

applying foam containing an agent capable of lowering the surfacetension of said foam liquid;

causing the foam to permeate the interstices of the sheet material bythe application of a pressure gradient thereacross;

removing the foam from the other side of said sheet material.

The process of the invention may be used to dewater an air permeablesheet material, or to apply treatment materials thereto.

One aspect of the present invention, therefore, provides a deliquifyingprocess for an air permeable sheet maerial which process comprises

forming a foam containing agent capable of lowering the surface of saidfoam liquid,

applying said foam to one side of an air permeable sheet material,

causing the foam to permeate the interstices of the sheet material bythe application of a pressure gradient and removing liquid and foam fromthe other side of said sheet material whereby the foam causes the liquidto be removed substantially from between the interstices of the sheetmaterial.

An alternative aspect of the present invention provides a process forapplying a reagent to an air permeable sheet material which processcomprises:

forming a foam containing said reagent,

applying said foam to one side of an air permeable sheet material,

applying a pressure gradient across said sheet material to cause thefoam to permeate the interstices of the sheet material,

and removing foam from the other side of the sheet material.

In one embodiment of the present invention, there is provided a processfor reducing the water content of air-permeable sheet material includingthe steps of:

1. applying a foam to wet air-permeable sheet material immediately priorto the drying step, the foam containing an agent capable of reducing thesurface tension of water

2. causing the foam to permeate the structure and interstices of theair-permeable sheet material; by applying mechanical means such asmechanical pressure in the nip of at least two rollers and/or a pressuregradient between one face of the sheet material and the other, all ofthese steps or any one of them being repeated if desired.

The residual water may be removed even more effectively by carrying outsteps 1. and 2. of the sequence described above, then blowing heated airof such volume and speed against one face of the wet air-permeable sheetmaterial that the stream of heated air penetrates to a substantialdegree through the sheet matrial, i.e. exits thereform on the opposingface at a speed and in a volume per minute which is at least 10% of thespeed and volume blown against the other face.

The process of the invention is also extremely suitable for the loweringof the water content of wet double layers of sheet material, e.g. of twolayers of textile fabrics.

This is particularly important because with a multiple layer processinge.g. of textile fabrics the process of the present invention provides atmany finishing stages a very substantial saving in processing costs. Theproblems inherent in conventional methods for the water level reductionprior to drying become more severe in the case multi-layer handlingsince, for example, the nip action of rollers becomes less efficient andmore complex, linear pressure in the nip due to the compressibility oftwo superimposed more or less open structures is smaller), and newproblems arise, e.g. the formation of undesirable patterns (moireeffects) and fibre entanglement between the two layers if the nippressures are as high as they have to be to at least come near theeffects obtainable with single layer processing. These advantages of thesystem become, of course, even more important if multilayer sheetmaterial such as 10 to 20 layers of e.g. gauze fabrics, or multiplelayers of sheet material with low physical integrity (such as non-wovensor paper) have to be processed.

The foam may be caused to permeate the interstices of the sheet materialand may subsequently be removed therefrom by virtue of a pressuregradient applied across the material.

In a particular embodiment of the present invention, a vacuum may beapplied to one side of the sheet material which serves to "pull" thefoam through the air permeable sheet material to be treated.

The invention further includes, therefore, a process which comprises thefollowing steps:

1. Applying a foam to one side of the air permeable sheet material to betreated said foam containing an agent capable of reducing the surfacetension of the liquid.

2. Causing the foam to permeate the structure and interstices of the airpermeable sheet material by causing a pressure gradient to form betweenthe two surfaces of the air permeable sheet material, whereby thepressure on the side to which the foam was applied is higher, to causethe foam to permeate said air permeable sheet material, providing a foamflow-constraining and equalizing substrate having in wet state a lowerair permeability than the wet air permeable sheet material, in intimatecontact with the surface of the air permeable sheet material not coatedwith foam, wherein the pressure gradient is of a magnitude sufficient tocause the foam to pass through both the air permeable sheet material andthrough the foam flow constraining substrate.

The air permeable sheet materials which may be treated according to thepresent invention comprises woven, knitted and non-woven textile sheetmaterial, paper at different levels of sheet formation (dewatering afterthe wet sheet has been formed, after dewatering treatments of otherkinds), sheets of loose fibres (fibre stock in the form of webs,oriented or non-oriented sheets of loose fibres, i.e. in a layer havinga thickness which is much smaller than the width, while the length isvery large compared to the width, such as roving, sliver, webs producedby carding etc.). Textile fabrics may be present in single or multilayerconfiguration. As many as 16 layers have successfully been treated bythe process of the present invention. Other airpermeable sheet materialwhich may be dewatered by the process described may comprise a bed orlayer of particulate matter, which is carried for instance on a porousconveyor belt (the foam flow-constraining substrate may serve as such,or it may travel on a porous endless belt).

In another typical embodiment, the air permeable sheet material may bequite thick, for example, a pulp sheet; initially such a layer may notbe air permeable per se due to the amount of liquid present: onapplication of the pressure gradient, surplus water is removed and thesheet material becomes air permeable. Thus, the air permeable sheetmaterials of the invention include inherently air permeable sheetmaterials capable of becoming air permeable on application of thepressure gradient.

The airpermeable sheet material may be thin, i.e. have a low thickness,or be three-dimensional in the sense that it consists or more than onelayer of a thinner material as for example a gauze.

The airpermeable sheet material may be structured, i.e. it may consistof or contain structural elements such as fibres or particles, clustersof fibres or particles with open spaces or voids between these elements,hereinafter referred to as "interstices". These structural elements maybe bonded together by bonding agents, by hydrogen or other non-covalentbonds, by covalent bonds, by mechanical interlacing or entanglement, orthey may not necessarily be held together, particularly in the case ofsheets or layers of particulate matter.

The air permeable sheet material may comprise natural material and/orsynthetic polymers. The sheet material may typically be less than 30 mmthick in the wet state, but thicker sheets may be treated if theairpermeability is sufficient to allow the foam to permeate thestructure at a reasonable rate and under the influence of the availablepressure gradient. The foam applied to the airpermeable sheet materialis preferably aqueous, but it may contain if desired non-aqueousliquids, e.g. in the form of an emulsion. The foam contains an agentcapable of reducing the surface tension of the foam liquid and in thecase of said liquid being water, said agent may be cationic, anionic,non-ionic, amphoteric surfactants (tensides), or simply a non-surfactantlowering the surface tension of water when added thereto, e.g. alcohols(mono or polyhydroxy compounds), amines and amides. In certain cases itis desirable to remove such agents after dewatering, e.g. during drying.A volatile agent may be used, i.e. an agent lowering the surface tensionof water which has a boiling point lower or close to the boiling pointof water, which is carried off by water vapour; alternatively an agentmay be used which decomposes at temperatures in the range of 50° to 100°C. (i.e. during drying) or at temperatures above 100° C., preferably nothigher than 200° C., during a heat treatement carried out during orafter the drying step. Mixtures of different types of agents loweringthe surface tension may, of course, be employed.

Such volatile or heat-decomposable agents are usually used only for thelast dewatering or washing step, since in intermediate steps it may bedesirable to re-use the liquid or foam/liquid mixture drained from theairpermeable sheet material, e.g. in the form of a system where lightlysoiled liquid is used in foam form for the dewatering or washing ofsheet material containing a higher concentration of soiling or pollutingagents, i.e. agents to be removed from the sheet material (counterflowwashing concept). The presense of an agent reducing water surfacetension in these cases is desirable because re-foaming (partial orcomplete, i.e. from a foam having a lower foaming ratio or from alargely air-free liquid) is necessary and should preferably be achievedwithout the addition of additional amounts of surfactants.

The foam may be produced in any convenience manner; e.g. static systems,which contain few, if any, moving parts, where foam essentially isproduced by blowing into the liquid to be foamed through fine orificesto introduce tiny bubbles into water at predetermined air to liquidrates, or dynamic systems, where air is beaten into a liquid by varioussystems involving rotating parts, e.g. rotating discs (usually serratedalong the circumference) arranged on a shaft, one of these discs movingclockwise, the next counterclockwise and so on, or other devices capableof introducing air into a liquid to produce a defined structure for thecells of the foam.

The size of foam cells should preferably be fairly uniform, i.e. verylarge bubbles should not be present in small cell-sized foam since sucha heterogeneous foam may give non-uniform and inconsistent results.Generally speaking the largest cells present in the foam applied shouldnot have a diameter larger than the thickness of the layer of foam to beapplied to the airpermeable sheet material and preferably it should beat most half the thickness of the layer. More uniform effects areobtained if the cell size is not larger than a quarter or preferably atenth of the foam layer thickness deposited.

The concentration of agents capable of reducing the surface tension inthe liquid before or during foaming obviously should be kept at theminimum necessary to obtain a foam of suitable foaming rate and foamstability.

The foaming rate is the ratio between the volume of the liquid afterfoaming to the volume of the liquid to be turned into a foam. A foamingrate of 10:1 thus means that the volume of the foamed liquid is tentimes the volume of the unfoamed liquid. Foaming rates between 200:1 and5:1 may be used, but a range between about 150:1 and 10:1 or preferablybetween 100:1 and 15:1 have been found most advantageous. The foamingrate obviously will determine the volume of foam to be applied if agiven amount of liquid is to be used in the form of foam to dewaterairpermeable sheet material. Thicker layers, i.e. higher foaming ratesare desirable if the thickness of the sheet material varies due to itsstructure or surface texture. All surface features of the sheet materialto be dewatered or treated should be immersed in the layer of foam toachieve uniform dewatering effects, and thicker layers of foam may beapplied if there is a considerable variation between the maximum andminimum thickness of the sheet material.

In one embodiment of the invention, the foam applied to the sheetmaterial to be treated is caused to permeate into and through thestructure and interstices between structural elements by causing apressure gradient to form between the surface to which the foam wasapplied and the side remote therefrom, the pressure being higher on thefoam-coated side. Pressure applied from the side of the sheet materialcarrying the foam, or vacuum applied to the reverse side, or both, willforce the foam to travel at substantially a right angle to the plane ofthe sheet material.

The use of vacuum has certain advantages over the use of pressure. It iseasier to apply in a well defined area on the side opposite the foamlocation, the vacuum applying means (e.g. a vacuum slot) may be indirect contact with the substrate with no loss of energy sinceessentially the vacuum acts only on the sheet material/substrate and thefoam lying on the sheet material, with little or no air seepage from theoutside.

Air pressure applied to the foam on the other hand is much moredifficult to direct exclusively onto the foam and through the sheetmaterial (some air will always be diverted due to the fact that thenozzle has to be above the surface of the foam layer). Foam is likely tobe blown off the surface of the sheet material instead of through it forthe same reason. Removal, collection and draining of the foam/liquidexiting after permeation is much more difficult with air pressure.Another important advantage of vacuum as a pressure gradient-producingmedium is the fact that a vacuum slot will stabilise the movement of thesheet material by holding it rather than causing it to flutter as astrong stream of air does. For these and additional reasons such as foambreakdown or a strong decrease of the foaming rate which can be producedby vacuum, but not (at least not to the same degree) by air pressure,and simple recycling of drained liquid/foam, the use of vacuum appliedto the side of the air permeable sheet material not carrying the foam isthe preferred method for creating a pressure gradient and causing thefoam to permeate into and through the sheet material.

The foam emerging from the downstream side of the sheet material is notidentical to the foam as applied, since for instance, its foaming ratiois decreased by the water removed from the airpermeable sheet material.Depending on the properties of the foam, it may also be lowered by thepermeation process. It may be further decreased (which in many cases isdesirable) by adjusting the stability of the foam to the minimum leveldesirable from the point of view of foam collapse between foamformation, foam deposition on the sheet and the time permeation starts.Passage through porous substrates may also affect the size of foam cellsand foam cell size distribution, i.e. the difference in the size of thesmallest and the largest cells. Material and agents removed by the foamfrom the sheet material may also affect the characteristics of theliquid or foam or foam/liquid mixture exiting from the sheet material.Generally speaking, it is desirable to have a low foaming ratio orsubstantially no foam in the vacuum slot, at least if the liquid is tobe discarded. But even if it is recycled, one may have better controlover the process if the drained foam or foam/liquid mixture is re-foamedto a predeterminable foaming rate.

In other cases it may be desirable to drain liquid essentially in theform of foam, i.e. to incorporate water removed from the sheet materialinto the foam permeating through it. In such cases the stability of thefoam applied and the foaming ratio (which is lowered by the liquiddrained from the sheet) may be suitably adjusted, i.e. the foamstability is increased, the foaming rate preferably being kept at such alevel that the foam can be reapplied if desired even without refoaming.In many cases it may be desirable to reduce the foaming rate tovirtually zero, i.e. to use conditions and equipment where liquidcontaining little or no air exits from the system. In this case, onewill reduce original foam stability.

In another embodiment of the present invention, a foam flow constrainingsubstrate may be disposed in juxtaposition with the air permeable sheetmaterial to support the same during the foam treatment. The foam flowconstraining substrate is preferably juxtaposed the air permeable sheetmaterial on the side remote from that to which the foam is applied. Inan alternative embodiment, however, the foam flow constraining substratemay be juxtaposed the air permeable sheet material on the side thereofto which the foam is applied.

Whichever embodiment is employed where a foam flow constrainingsubstrate is used, it is preferably a sheet material having thefollowing characteristics:

1. Ensuring an essentially uniform permeation of air liquid and foamthrough interstices or pores in the sense that these pores aredistributed evenly over the surface of the substrate and that themaximum diameter or cross section of the pores are predeterminable andknown magnitude; if the size of the pores is not geometrically definablesuch as for instance in the case of a non-woven fabric then the air andfoam permeabilities may be determined by a large number of small poresand not by a relatively small number of large pores.

2. Ensuring that the air permeability of the substrate material is atthe most equal to that of the air permeable sheet material to be treatedand preferably, at least 10% lower than the air permeability of the airpermeable sheet material.

3. Ensuring that the maximum diameter of these pores is preferably atthe most, 50 microns, and more preferably not greater than 30 microns.

The uniformity of the maximum pore size in the foam flow constrainingsubstrate results not only in constraint, but also in equalisation ofthe flow of foam through the sheet material and said substrate.

The substrate may be a woven fabric or a non-woven web. The constructionof the fabric or web should be sufficiently stable to retain the porecharacteristics in use.

This is usually easier to achieve in the case of more planar, i.e. lessthree-dimensional configurations as opposed for instance to knittedstructures, which are not only more open, but tend to become distorted(with some pores becoming larger) if exposed to stress. Knitted fabricsfor this reason were found to be less suitable, unless the configurationof interlacing yarns and fibres is sufficiently stabilised by blockingfibre-to-fibre and yarn-to-yarn movement (such blocking may also beuseful or even necessary in the case of unstable woven fabrics or webs),and provided airpermeability and maximum pore diameters can be held atthe levels specified above and below.

The pores or interstices through which the pressure gradient causes thefoam to permeate through the airpermeable sheet material and the foamflow-constraining substrate, may be essentially round or square as inthe case of a filter fabric, where pore size and pore shape isdetermined by the open space lying between yarn intersections (the yarnbeing very compact), or they may have oblong shapes, i.e. they may beformed by single fibres arranged in relatively parallel configurations,such as fibres forming a yarn with a relatively small number of turnsper inch. It has been found that woven fabrics consisting in at leastone direction of a yarn with a very low twist factor (i.e. few if anyturns per inch), where fibres (preferably filament fibres) due to thelow number of turns are arranged in an essentially parallelconfiguration relative to each other and again due to the low twistfactor rather form an essentially two-dimensional ribbon or band insteadof a three-dimensional yarn with a more or less circular cross-section,are particularly suitable among woven fabrics. Filter fabrics, i.e.fabrics of very tightly woven structures with very compact yarns aresuitable due to the very accurate maximum pore size and the wearresistance of such fabrics. While pore size in the case of filterfabrics is defined by the open space between yarn intersections i.e. bythe yarn diameter, yarn construction and fabric construction, it isdetermined by the spacing of the essentially parallel filaments of theribbon-like low or no twist yarns in the case of the other type of weavementioned.

In many cases, other woven fabrics, i.e. fabrics containing either lowor no-twist yarns, or filter fabric yarns, may be used provided theirairpermeability is at most equal, preferably at least 10% lower thanthat of the sheet material to be dewatered, and provided maximum poresizes are less than 50, preferably less than 30 microns. Cellulosic,cellulosic blend or synthetic fabrics have under these conditions givenadequate dewatering effects.

Filter fabrics made of synthetic filament yarns with a mesh aperture ofat most 50, preferably at most 30 microns are suitable for achievinggood dewatering effects. If stationary filter plates are used toconstrain foam flow, best results are obtained if the maximum porediameter is 40 microns, preferably 30 microns. Airpermeabilities of atmost 4000, preferably at most 2500 liters/square meter/second giveacceptable effects in the case of filter fabrics.

In the case of woven fabrics consisting of yarns and fibres which do notgive fabric structures with porosity features as well defined as filterfabrics, airpermeability has been found to be the best criterion. Wovenfabrics should have an airpermeability (measured in wet state at leastif water-swellable fibres are present) of at most 250, preferably atmost 200 liters per square meter per second (determined at a pressureequal to the weight of a water column of 20 centimeters). Woven fabricshaving an airpermeability of 100 l/sq.m./sec. or even 10 l/sq.m./sec.have given excellent results.

Nonwoven structures for use as the foam flow constraining substratehaving a maximum airpermeability of at most 2000, preferably at most1000 liters per square meter per second give acceptable dewateringeffects. It is preferred that the fibres of the web should be suitablyspaced, the pores (i.e. open space between fibres) should be distributedover the web in sufficient uniformity and the configuration of theinterstices between fibres which define pore size should be sufficientlystable (i.e. if it does not change-affecting pore size anduniformity-under the influence of the pressure gradient and/or actualuse).

Uniformity of pore distribution over the area of the substrate and ofmaximum pore diameters is important because the foam flow-constrainingsubstrate not only serves to constrain the flow of foam by causing thefoam to flow through a large number of pores with a relatively uniformmaximum pore diameter, but also to equalise the volume of foam forcedthrough the sheet material over its entire surface and the substrate bythe pressure gradient in the sense that the thickness of the foam layeris reduced uniformly over the surface of the airpermeable sheetmaterial, i.e. that zero foam layer thickness is reached at virtuallythe same time all over the surface of the sheet material. If in certainplaces foam would permeate substantially faster than in others,dewatering effects could become non-uniform because due to the differentflow-through properties of foam and air, the areas where zero thicknessof the foam layer is reached first would act as by-passes, i.e. theresidual foam on the other areas would permeate more slowly orincompletely, thus affecting the removal of water from the sheetmaterial in those areas. The foam flow-constraining substrate thusserves both to channel uniformly the flow of foam and to ensure that thepressure gradient, the flow of foam through the sheet material and hencethe dewatering effect is uniform over the surface of the airpermeablesheet material even if the latter due to its structure or configurationshould have non-uniform air or foam flow-through properties.

The foam flow constraining substrate may be in close contact with thesheet material to be dewatered, i.e. there should be no open space orgap between the sheet material and the substrate except open spacedetermined by the surface texture of the two sheets, hence the pressuregradient should be acting through both sheets without any appreciableamount of air entering between the edges of the two sheets in the caseof vacuum, or air escaping between the sheets if air pressure causes thepressure gradient to form.

In the preferred mode of the invention the airpermeable sheet material,to which a layer of foam is applied travels in close contact with thefoam flow-constraining substrate, which thus carries the sheet material,for instance over vacuum slots producing the pressure gradient and whichdraws the foam lying on top of the airpermeable sheet material throughthe latter and through the substrate underneath.

This system not only has the advantage that an airpermeable sheetmaterial having little or no mechanical integrity of its own may betreated easily, but that a delicate sheet material (i.e. materialsensitive to damage by friction) is not caused or allowed to rub againststationary surfaces such as the edges of a vacuum slot. At the sametime, the system is very versatile in the sense that optimum dewateringeffects on sheet material of a wide range of construction,configuration, airpermeability and bulk may be achieved simply by usinga suitable foam flow-constraining substrate, by applying a suitable foamand adjusting if necessary the pressure gradient.

Foam flow-constraining substrates may comprise natural or syntheticfibres, blends or inorganic material such as glass or metal fibres orthin wires (wire mesh) provided it has an airpermeability lower than thesheet to be dewatered and preferably a maximum pore size (mesh aperture)of at most 100 micron, preferably lower than 50 microns or even lowerthan 30 microns. Perforated metal, perforated plastic sheet material, orwoven material gauzes may be used provided the specifications mentionedabove apply.

Such substrates may be arranged in the form of endless belts, or ofrotary screens. Stationary filter plates may also be used if they meetspecifications as regards maximum pore size, but the friction createdbetween the sheet material and the filter plate by the movement of thesheet material and enhanced by the pressure gradient may bedisadvantageous. The permeability to air of the foam flow-constrainingsubstrate should as mentioned above be lower than the permeability toair of the wet sheet material to be dewatered (in the case of substratesconsisting of or containing water-swellable fibres, one should determinethe airpermeability in wet state).

Substrates having a very much lower airpermeability than the sheetmaterial to be dewatered may give very good dewatering effects; in factin most cases, for a given type of substrate, dewatering effectsincreased (i.e. residual water content decreased) with decreasingairpermeability of the substrate as is shown in Table 1.

It is of course not possible to correlate directly types of fabricsdiffering basically as regards their foam flow-constraining features,e.g. filter fabrics (where pores are defined by the yarn diameters andyarn spacing) to woven fabrics where the spacing of for instance lowtwist filamentous fibre material arranged in ribbon-like fashiondetermines air and foam flow properties, or to nonwoven structures wherethe orientation, spacing and configuration of fibres and fibreintersections determine pore size. Furthermore, not only theairpermeability, but to an even larger degree the pore size mayinfluence the degree of water removal for a given sheet material.

In the case of filter fabrics (polyester, polyamide or other syntheticfibres), where air and foam fIow characteristics as well as pore sizeare almost exclusively defined by the diameter of the yarns used andhence the mesh count, dewatering performance follows very closely themesh aperture and to a slightly lesser degree airpermeability as isshown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________    Residual Water                                                                          Filter Fabric No.                                                   After Dewatering                                                                        31   32   46   39   44   37   41                                    (% owf)   130  140  170  180  185  195  195                                   __________________________________________________________________________    Mesh aperture                                                                           25   26   100  58   80   53   80                                    Mesh count                                                                              184.5                                                                              165.7                                                                              58.5 110.5                                                                              74.5 120  81.1                                  Yarn diameter/cm                                                                        0.030                                                                              0.035                                                                              0.070                                                                              0.033                                                                              0.054                                                                              0.030                                                                              0.043                                 Open Surface %                                                                          19   173/4                                                                              3.5  40   35.75                                                                              41   42.5                                  Air-Permeability                                                                        2100 1250 4400 4450 4400 5050 6000                                  1/m.sup.2 /s)                                                                 Water Permeability                                                                      485  265  780  --   770  850  950                                   (1/m.sup.2 /s)                                                                __________________________________________________________________________

The data set out in Table 1 above shows that among filter fabrics thosewith a mesh aperture higher than 30 removes substantially less waterthan fabrics with a mesh aperture below 30. The fabrics having thelowest mesh aperture also were those with the lowest air and waterpermeabilities, the highest mesh count and the lowest open surface.

Such correlation between dewatering effect, mesh aperture, airpermeability and mesh count and open surface of filter fabrics andfilter plate was found for widely different airpermeable sheet materialranging from tissue paper to nonwoven webs to cotton broadcloth andeight to sixteen layers of cotton gauze. In addition to a mesh apertureof at most 30 microns, a mesh count above 100, preferably above 150, anopen surface below about 25, preferably below 20 and airpermeability ofless than 3000 l/sq. m./sec. (liters per square meter per second) arefactors ensuring a high rate of dewatering.

In certain cases one may, of course, have to compromise as regards thedewatering effect/airpermeability or open area ratio, e.g. if sheetmaterial is moving extremely fast, if it contains very high amounts ofwater or if for any other reason high permeability of the foamflow-constraining substrate is desirable.

One may for instance prefer to use a more open structure of filter clothat least in preliminary washing steps to achieve a high flow-throughrate.

In the case of woven fabrics with characteristics not as well defined asin filter fabrics, the pore size as mentioned earlier may be determinedas much or more by fibre to fibre spacing as by yarn intersectionspacing. But even among fabrics of widely different constructions, thestructures with the lowest airpermeability give the best dewateringeffects as is shown in Table 2.

                  TABLE 2                                                         ______________________________________                                             Fabric    Fibre Material                                                                             Airperm.                                                                              Resid. Water                              No.  Constr.   Remarks      1/m /sec.                                                                             Content %                                 ______________________________________                                        10   Ribs      Nylon, filling                                                                              10      95                                                      yarn with extre                                                               mely low twist                                                                factor                                                          3   Twill     Cotton        15     120                                       11   Plain     Polyamide para-                                                                            200     130                                            Weave     chute cloth,                                                                  filament yarns,                                                               very light weave                                               13   Plain     Polyester,   250     150                                            Weave     staple fibre                                                                  yarn                                                           18   Broad-    Cotton       280     175                                            Cloth                                                                    14   Plain     Polyester    300     195                                            Weave                                                                         similar                                                                       to No. 13                                                                 5   Nonwoven  Polyester    1200    160                                       ______________________________________                                         *Fabric dewatered: Nonwoven, airtangled.                                 

Since there are hardly any methods known for defining, let alonedetermining "pore aperture" for fabrics of widely differentconstruction, yarn characteristics, and yarn configurations, theairpermeability (determined in wet state if water-swellable fibres arepresent) is the most meaningful and universally applicable ratingcriterion as regards dewatering effects obtainable.

Another method is the so-called bubble-point test used by producers offilter cloth to define "nominal pore size".

In the case of woven fabrics, for instance a nominal pore size (asdetermined by the bubble point test) of at most 30, preferably at most20 gives the best dewatering effects if these fabrics are not filtertype fabrics.

It is also a useful method for evaluating the effect of mechanical orother treatments which may be applied to improve the dewateringproperties of a given fabric (such as calendering, and shrinking).

Nonwoven fabrics have been used with average results for dewatering,provided the configuration of fibres and fibre intersections are wellfixed by proper bonding to avoid distortions leading to uneven pore sizedistribution, and provided the web is uniform as regards pore size andpore distribution in the material. Such nonwovens which may be used togive average dewatering effects as shown in Table 2, since the averagepore size may have much higher airpermeability than conventional wovenfabrics (but usually lower than filter fabrics).

In preferred embodiments of the present invention, the characteristicsof the foam should be selected such that:

1. a foaming rate of the foam applied to the surface of the airpermeablesheet material of 300:1 to 5:1 may be used; better results may beobtained if this range is between 150:1 to 15:1, with about 80:1 to 20:1being the optimum range for most applications.

2. The volume of foam applied to the sheet material and caused topermeate through it should be such that the foaming rate calculated fromthe weight of liquid initially applied in foamed form, of this foam andthe liquid removed from the airpermeable sheet material is 10% to 80%,preferably 30% to 60% lower than the foaming rate of the foam originallyapplied. It is, of course, desirable to use as little liquid for thedewatering as possible. Depending on the characteristics of the sheetmaterial to be dewatered (evenness of the surface, thickness, openness,amount of water to be removed, time available for permeation, pressuregradient available), a high, medium or low foaming rate may be moreadvantageous.

3. In order to get good dewatering effects at low add-on and low foamvolumes existing in the system, foam stability levels, foam volumesapplied, foaming rates of the foam applied and pressure gradients usedas well as the characteristics of the foam flow-constraining substrateshould be selected in such a way that the actual foaming rate of thefoam/liquid mixture exiting from the foam flow-constraining substrate isless than 50%, preferably less than 20% of the foaming rate of the foamoriginally applied to the surface of the airpermeable sheet material.

While the change of the foaming rate specified in 2. may be calculated,the change specified in this paragraph is actual, i.e. to be determinedby measuring the volume and the weight of the foam/liquid mixture beforeand after permeation.

This reduction of the actual foaming ratio may be increased by using afoam of low stability, a relatively low foaming rate and pressuregradients and foam flow-constraining conditions conductive to arelatively high degree of foam breakdown.

4. If an even lower foaming ratio or practically no foam is desirable atthe exit end of the system, the foaming rate may be further reduced bycarrying the foam/liquid mixture under the action of the pressuregradient, preferably vacuum, through a pipe or tube equipped with atleast one venture having at least one segment where the cross-section ofthe tube or pipe narrows suddenly by at least 5% preferably at least 25%of the cross-section. Virtually untapered narrowing sections, i.e.sections where the cross section narrows rather abruptly are moreadvantageous than long tapered sections.

5. Good dewatering effects are obtained while lowering foaming ratios,i.e. the volume of foam leaving the system, by adjusting the stabilityof the foam applied to the airpermeable sheet material to such a levelthat this stability expressed in terms of foam half-life is reduced byat least 25%, preferably at least 50% by the passage through the sheetmaterial and the associated foam flow-constraining substrate and by thedilution produced by the liquid removed by the treatment from the sheetmaterial. This particularly applies if vacuum is used to produce apressure gradient.

"Half-life" as applied to foam in this specification means the timeafter which the volume of a foam put into a beaker at 20° C. has droppedto 50% of the original volume, half of the foam volume thus havingcollapsed.

Some of the reduction of foam stability may be produced by the passagethrough the porous sheet material and the substrate, while some foamstability loss is due to the dilution occuring inside the wetairpermeable sheet material. In most cases foam stability loss,irrespective of its cause, is a useful criterion for the selection ofprocessing conditions, in particular of the stability of the foamoriginally applied. The stability is determined not only by the type andconcentration of the agent reducing surface tension present in the foam,but also by the foaming rate and to some degree by the shape and size offoam cells, in particular by their maximum size. This gives a wide rangeof options as regards the formulation of the foam and the optimizationof the formulation from the point of view of other criteria mentioned.

The magnitude of the pressure gradient depends on processing conditionsand the sheet material to be treated (i.e. time available forpermeation; volume of foam applied per area, e.g. per square centimeter;structure, weight, density, thickness of the sheet material; and amountof liquid to be removed). Practically all the foam applied to thesurface of the sheet material should be caused to permeate into,preferably all through, the entire thickness of the sheet material.

The time of exposure of the airpermeable sheet material, to which foamhad been applied, to the pressure gradient preferably is such thatvirtually all of the foam applied is caused to permeate through saidsheet material. If, for some reason, a layer of foam is to be left, orif the action of the pressure gradient is to be terminated before allthe foam has been removed from the surface to which it had been applied,the residual layer of foam may be removed, for instance, by scrapping orby suction.

Permeation of the foam through the sheet material under the action ofthe pressure gradient may proceed in one or several steps, with one orseveral applications of foam to the surface of the sheet material to betreated, with the same or a different type and the same or a differentmagnitude of the pressure gradient causing permeation of the foam. Asmentioned before, the preferred method for causing permeation consistsin applying vacuum to the wet airpermeable sheet material through thefoam flow-constraining substrate, which is in close contact with saidsheet material and which by the action of the vacuum and the air-poreplugging action of the foam layer present on the surface of theairpermeable sheet material, is even more tightly contacted with saidsubstrate.

Vacuum for instance may be applied to the system by passing the foamflow-constraining substrate and the superimposed airpermeable sheetmaterial across one or several slots, such a "vacuum slot" comprising anenclosed area which is connected through a tube, pipe or duct to avacuum-producing pump. Multiple vacuum slots may be arranged in ahorizontal plane, a curve (preferably convex) or in a rotating drum, thesheet material and the underlying substrate preferably travellinghorizontally or at most at an angle of 90°, preferably at most 60° tothe horizontal plane. While the most advantageous configuration consistsin applying the pressure gradient, in particular vacuum, to the foamflow-constraining substrate having a lower and preferably a more evenairpermeability than the airpermeable sheet material, and through thissubstrate to the airpermeable sheet material, one may if desired applyfoam to the foam flow-constraining substrate, which travels (preferablywith the same speed) in close contact on the wet airpermeable sheetmaterial, and apply the pressure gradient, in particular vacuum in sucha way that the foam is made to permeate through the substrate, thenthrough the underlying sheet material to dewater the latter. Thisconfiguration as an alternative to the preferred one where the foam isapplied to the airpermeable sheet material, may in certain cases also beused for the washing application described below, at least in some of aseries of in-line dewatering steps. Dewatering effects are, however,inferior to those obtained by applying the foam to the air/permeablesheet.

The process according to this invention may also be used to removeagents from the air/permeable sheet material. Such agents may bechemical agents, particulate matter, liquids, solids or mixtures of suchproducts including impurities of undefined composition. In these cases,the foam applied to the surface of the sheet material (or the substrate)acts as washing medium, which removes undesirable agents and at the sametime dewaters the sheet material so that a second step under the same ordifferent conditions will be more effective as regards the agent removaleffect. The air/permeable sheet material may be dry when foam is made topermeate it for the first time to remove agents, or it may be wet as inthe case of dewatering. The foam applied may contain surfactantsparticularly suitable for removing the undesirable agents present,and/or it may contain compounds capable of neutralising, emulsifying ordispersing the undesirable agents present in th sheet material.

As in the case of dewatering, multiple treatments according to theinvention may be carried out in the same or in a differentconfiguration, under the same or different conditions as regards thetype, composition and properties of the foam used, the pressure gradientemployed, etc. To obtain maximum cleaning effects, it is important tooperate under conditions ensuring good dewatering effects. A furtheraspect of of the present invention is the inclusions within the foam ofagents which interact with the airpermeable sheet material or withmaterial carried therein, "interacting" meaning reacting chemically withsaid material or components thereof, forming covalent or non-convalentbonds (such as hydrogen or Van der Waals bonds) or just agents fordeposited in the interstices of the said sheet material.

Such interaction treatments may be carried out independently or incombination with agent removal and dewatering treatments.

The foam may be applied to a dry air permeable sheet material, inparticular foam may be forced into the dry airpermeable sheet materialto form an inner interface under conditions (in particular as regardsthe absorbency of the substrate for the liquid forming the foam cells),which enable foam transit through the substrate. This is particularlybeneficial in cases where

(i) foam collapse by water adsorption by the material of theairpermeable sheet material is to be prevented (i.e. if the watercontent of the latter in the case of removal of undesirable agents orthe application of agents is relatively low (dewatering thus beingnecessary only after agent removal or agent application);

(ii) if for other reasons a minimum amount of water is to remain in theairpermeable sheet material;

(iii)if interaction with the material of the airpermeable sheet isdesired to take place within its structure, i.e. if interaction is toproceed at inner interstices (and if desired also at the surfaceinterface), foam may be forced into the dry airpermeable sheet materialto form an inner interface under conditions (in particular as regardsthe absorbency of the substrate for the liquid forming the foam cells),which enable foam transit through the substrate.

In these circumstances, the foam thus applied may contain agents capableof producing the interaction desired, or if such agents are appliedsubsequently, interaction will take place not only at the surface towhich such agents are applied, but also internally at any innerinterfaces which may be formed. Foam transition conditions aredetermined and achieved by causing a sheet of foam of uniform thicknessto permeate through the airpermeable sheet material under the action ofa pressure gradient, the sheet material being exposed to the action ofthis pressure gradient only for such a period of time until the firstfoam cells appear on the opposite side of the sheet material.

The foam flow-constraining substrate may be cleaned in order to removeparticulate or fibrous debris carried by permeating foam from theairpermeable sheet material into the substrate or already present in thefoam when it was applied, by reversing the flow direction (using foam,water, spraying of water, air blown against the substrate) after thesubstrate has been separated from the airpermeable sheet material.

Water, foam or air is thus pressed through the substrate from the sidewhich had not been in touch with the sheet material, i.e. where thepressure had been lower during the treatment according to the presentinvention. If water/soluble material has to be removed from time to timeor after each cycle of foam permeation, washing may either proceed byreversing the flow direction or using the same direction as before. Ifsoiling or clogging by debris is very severe, one may use different foamflow-constraining substrates in-line, i.e. transfer the airpermeablesheet material from one substrate to another between treatmentsinvolving foam permeation.

Following is a description by way of example only of methods of carryingthe invention into effect.

The following data demonstrates the strong beneficial effects of theprocess of the present invention.

In the examples, the following explanations and abbreviations will beused.

FFCS: Foam flow constraining substrate

APSM: Air-permeable sheet material

MEF (APSM)

Blott-Paper (APSM)

Tissue (APSM)

Gauze (APSM) 8 layers of surg. gauze, bleached and scoured, . . .

Broadcloth (APSM)

Foam Formulations and Specifications ("Foam")

Blow ratio: volume of foamed liquid to volume of liquid before foaming

Formulation: Agents present in liquid to be foamed

Formulation A: 2 grams/litre of nonionic surfactant (Sandozin NIT conc,Sandoz)

Formulation B: 1 gram/litre of same nonionic surfactant

Formulation C: 0.2 grams/litre of same surfactant

Foam Volume: Volume of foam (in ml) applied to surface of APSM beforeapplying pressure gradient volume in ml per dm2.

Dewatering Effect:

Bath content of APSM after applying foam, creating a pressure gradientcausing the foam to permeate through the APSM and the FFCS, anddetermining and comparing the weight of the APSM sample after thistreatment to its weight before the treatment, expressed in %owf (% onthe weight of the fabric).

Residual Water Content:

Water content of APSM after dewatering treatment (as opposed to"original water content", i.e. water content before dewateringtreatment).

EXAMPLE 1 Effect of Presence of Foam in Multi-Layer Substrates (WovenFabrics)

Processing and handling of fabrics in the tests: Two or moresuperimposed layers of the textile fabrics mentioned were treated in wetstate (pure water) as follows

(a) Hard squeeze in nip between rollers, double passage, i.e. manglingrepeated

(b) same, light squeeze, one and two passages,

(c) same, but foam applied to the layers of fabric (between layers)before same squeeze as in (b), only one passage.

The effects obtained are expressed in grams of fabric plus residualwater per 100² cm.

The presence of agents lowering the surface tension of water per se hasbeen found to increase the effect of known mechanical water removalsystems such as squeezing in a nip etc., particularly if thewater-removing treatment has to be mild from the point of view ofmechanical action, e.g. mechanical pressure applied to the sheetmaterial.

Applying such agents in a foam bath will, however, further reduce theresidual water content to a very substantial degree as shown in thefollowing Table 3.

                  TABLE 3                                                         ______________________________________                                        Non-woven, 2.15 oz/sq yard, 100% rayon                                        ______________________________________                                        Sample 1  two layers of the non-woven padded in                                         pure water, squeezed gently in mangle                               Sample 2  padded in water containing agent                                              capable of lowering surface tension of                                        water, squeezed on same mangle in same                                        way as Sample 1                                                     Sample 3  same treatment as for Sample 2, but                                           foamed bath (same composition as padded                                       bath) fed between the two layers of                                           non-woven before squeezing                                          ______________________________________                                                               Residual Water Content                                 ______________________________________                                        Sample 1               200%                                                   Sample 2  (0.25% surfactant)                                                                         130%                                                   Sample 2  (0.01% surfactant)                                                                         180%                                                   Sample 3  (0.25% surfactant)                                                                         110%                                                   Sample 3  (0.01% surfactant)                                                                         160%                                                   ______________________________________                                    

Since in certain cases it is undesirable to have residual surfactantspresent on the sheet material after drying, it has been found that insuch cases one may use surfactants decomposing under the influence ofdrying temperatures, or carried off by the evaporating water, orsurfactants which have an evaporation temperature not much higher thanwater.

                  TABLE 4                                                         ______________________________________                                                  100% cotton                                                                            100%                                                                 broad cloth                                                                            cotton voile                                                                             cotton gauze                                              (2 layers)                                                                             (2 layers) (16 layers)                                     ______________________________________                                        (a) Hard squeeze                                                                              4.12    g    2.32  g    9.3   g                                   2 passages                                                                (b) Light squeeze                                                                             5.2     g    3.84  g    11.7  g                                   one passage                                                               (b) Light squeeze                                                                             5.12    g    3.85  g    11.62 g                                   two passages                                                              (c) (b) treated 4.5     g    3.09  g    9.9   g                                   with foam                                                                     one passage                                                                   of treatment                                                                  (b)                                                                       ______________________________________                                    

The treatment (c) of a sample given the nip treatment (b) followed bythe same nip treatment in presence of a bath of foam thus gave aresidual water content considerably lower than either treatment (b)alone or the repeating of treatment (b), i.e. the presence of the foamin the fabrics during the squeezing treatment improved the squeezingeffect very substantially even though the treatment with foam hadincreased the water content beyond that of the wet material used for thetest.

EXAMPLE 2 Influence of Air Pass-through Treatment: Woven MultilayerSubstrates

The same samples as in Table 3 were after squeezing treated for 10seconds thereafter with a relatively slow stream of air blown againstone face of the sandwiched fabrics.

                  TABLE 5                                                         ______________________________________                                                  Broadcloth                                                                             Voile      Gauze                                                     (2 layers)                                                                             (2 layers) (16 layers)                                     ______________________________________                                        (b) one passage 5.2    g     3.88 g     11.72                                                                              g                                    through nip                                                               (c) one passage 4.5    g     3.09 g     9.9  g                                (b) after squeezing                                                                           4.95   g     3.60 g     11.5 g                                    treated with                                                                  air (room)                                                                    temperature)                                                              (b) after squeezing                                                                           4.8    g     3.3  g     11.5 g                                    treated with                                                                  air of 32° C.                                                      (c) after squeezing                                                                           4.38   g     2.84 g     10.05                                                                              g                                    treated with                                                                  air (room                                                                     temperature)                                                              (c) after squeezing                                                                           4.28   g     2.42 g     9.94 g                                    treated with                                                                  air (32° C.)                                                       ______________________________________                                    

These results show that the short treatment with air gives surprisingresults even if the air is at or only slightly above roomtemperature--irrespective of the number of layers present and eventhough rather low air speeds are used.

In some cases water levels are reached even under these very mildconditions, which are comparable to these obtained by very hardsqueezing. Higher air temperatures such as 60° to 80° C. and somewhathigher air speeds (yet well below the very high speeds used in nozzlesas recommended by certain equipment manufacturers) do of course giveeven better results even at shorter treating times. Air temperatures of40° to 80° C. are available at low cost from heat recovery systems oftenter frames, curing ovens or other thermal treating equipment. Air orwater at such temperatures was considered to be of little use hitherto.

EXAMPLE 3 Influence of Presence of Foam on Squeezing Effect: MultilayerNon-woven Substrates Procedure

Non-woven substrates (rayon, entangles) were wetted in an aqueous bathcontaining small amounts (0.2 g/liter) of a non-ionic detergent. Controlsample A was squeezed hard twice in sandwich form in the nip of apadding mangle. Control Sample A' was squeezed lightly in sandwich formin the nip of a mangle.

Sample B₁ was treated exactly as samples A, but after the squeezing inthe nip the same bath in foamed form was sucked through the squeezedfabric by means of a vacuum slot.

Sample B₂ was again treated in sample A, but a foamed bath of the samecomposition was fed into the space between two layers of the squeezednon-wovens before the sandwich entered the same nip as for sample A,i.e. during the mechanical treatment (squeezing) additional liquid infoamed form was present in the wet non-wovens.

Sample B'₁ was treated exactly as sample A', but after the lightsqueezing the foamed bath was sucked through the two layers by means ofa vacuum slot.

Sample B'₂ was treated exactly as sample A', but after the squeezing,the foamed bath was introduced between two layers of the squeezednon-wovens before passing the foam filled sandwich through the same nipas for sample A'.

                  TABLE 6                                                         ______________________________________                                        Air treatment: 5 seconds, air temperature 42° C.                                                     % Water                                                                              % Water                                        Foaming                 retained                                                                             after Air                                Sample                                                                              Rate     Treatment      owf    Treatment                                ______________________________________                                        A     --       hard squeeze,  120%   --                                                      2 passages                                                     B.sub.1                                                                             30:1     same, then foamed                                                                            120%   100%                                                    bath sucked through                                            B.sub.2                                                                             80:1     same squeeze   125%   100%                                                    sandwiched/foam                                                               inserted/squeeze as A                                          A'    --       light squeeze  230%   --                                       B'.sub.1                                                                            25:1     same, then foamed                                                                            135%   110%                                                    bath sucked through                                                  50:1     same           120%   100%                                           70:1     same           110%    70%                                     B'.sub.2                                                                            25:1     same squeeze sand-                                                                           120%   110%                                                    wiched/foam                                                          50:1     inserted/squeezed                                                                            110%   100%                                                    as A'                                                          ______________________________________                                    

Table 6 shows that the sucking of the foamed bath through the wetmaterial may reduce the water content by more than 50% (even though thefoam actually adds water to the water already present) and the feedingof the foamed bath between two wet fabrics before squeezing also reducesthe water content even though here again the foamed bath actuallyincreases the total amount of water present. The table also shows that avery short treatment with low temperature air will further markedlyreduce the water content.

A very important step of the procedure is to insert foamed liquidbetween layers of wet air permeable sheet material, and then causing thefoam to penetrate the sheet structure and remove liquid by passing thelayers with foamed liquid sandwiched between the layers through the nipof pressure rollers, i.e. rollers running in contact under adjustablepressure.

The application of the foam may be by known methods (knife, roller, kisscoating, from a trough or from perforated tubes to one or multilayeredsheet material such as fabrics--woven, knitted, non-woven--paper, airpermeable sheets of foam etc.).

The foam may be applied from one side, from both sides or between layersof the sheet material. The foamed liquid may be aqueous, containingsmall amounts of foaming agents, or it may contain agents such as foamstabilizers, agents destabilizing foams at elevated temperatures, andfinishing agents. It may be applied cold or have a temperature aboveroom temperature. In certain cases non-aqueous liquids may be used.

Known systems capable of removing water from wet material may be used.Not only may the application of the foam be integrated into thepermeation step, but the permeation process may be integrated into theliquid elimination process. One may for instance apply foam betweenlayers of multilayered sheet material (e.g. two, four or up to twentylayers of fabrics, the foam usually being applied between middlelayers), and then the material passed through the nip of a mangle,forcing the foam into the structure and eliminating liquid in the sametreatment.

EXAMPLE 4 Influence of Presence of Foamed Bath on Water Removal(non-Wovens)

                  TABLE 7                                                         ______________________________________                                                          % Water                                                     Samples Foaming   retained owf                                                                              Treatment                                       ______________________________________                                        A       --        110%        hard squeeze                                    B.sub.1 30:1      100%        same, then foam                                                               sucked through                                  B.sub.2 80:1      110%        hard squeeze,                                                                 foam fed into                                                                 sandwich, same                                                                hard squeeze                                    A'      --        230%        light squeeze                                   B'.sub.1                                                                              25:1      100%        same, foam                                              50:1       90%        sucked through                                          70:1       85%                                                        B'.sub.2                                                                              25:1      110%        light squeeze,                                                                foam                                                    70:1      105%        fed into sandwich,                                                            same light squeeze                              ______________________________________                                    

EXAMPLE 5

Water vs foam: Water sucked through APSM vs same volume of water infoamed form sucked through same APSM

                                      TABLE 8a                                    __________________________________________________________________________    Dewatering Effect in % owf                                                    FFCS    No. 10 No. 10 No. 10 No. 10                                           APSM    Gauze (8)                                                                            MEF    Blott-P.                                                                             Tissue                                           __________________________________________________________________________    Formulation.sup.(1)                                                                   (11)                                                                             115%                                                                              (11)                                                                             120%                                                                              (11)                                                                             120%                                                 Water   (27)                                                                             115%                                                                              (118)                                                                            110%                                                                              (104)                                                                            200%*                                                sucked through        (118)                                                                            120%                                                 same water.sup.(1)                                                                    (11)                                                                             90% (11)                                                                             80% (11)                                                                             95% (10)                                                                             78%                                           sucked through                                                                        (27)                                                                             90% (118)                                                                            80% (104)                                                                            150%*                                                as foam (60:1)        (118)                                                                            90%                                                  Strong  (11)                                                                             110%                                                                              (11)                                                                             120%                                                                              --     (10)                                                                             138%                                          Mangling                                                                              (27)                                                                             110%                                                                              (118)                                                                            120%                                                        __________________________________________________________________________     .sup.(1) Formulation A                                                        *7 layers, other test with one layer                                     

                                      TABLE 8b                                    __________________________________________________________________________    Influence on Surfactant in Water                                                                           No. 10                                                                        Fibre Stock                                      FFCS    No. 10 No. 10 No. 10 (2 layers of                                     APSM    Blott P.                                                                             MEF    Gauze (8×)                                                                     surgical cotton)                                 __________________________________________________________________________    Plain water                                                                           (11)                                                                             160%                                                                              (11)                                                                             280%                                                                              (11)                                                                             130%                                                                              (15)                                                                              280%                                         sucked through                                                                Water + Surf.                                                                         (11)                                                                             120%                                                                              (11)                                                                             110%                                                                              (11)                                                                             110%                                                                              (15)                                                                              340%                                         (Form. A)                                                                     sucked through                                                                Form. A (11)                                                                             90% (11)                                                                             80% (11)                                                                             90% (15)                                                                              135%                                         foamed (60:1)                                                                 sucked through                                                                __________________________________________________________________________

EXAMPLE 6 Influence of Mesh Aperture of the FFCS (Test 109)

The influence of the mesh aperture of different FFCS on dewateringeffects obtained on different substrates was investigated.

FFCS: Filter plates in Buchner funnels as model for FFCS

APSM:

Blotting paper (numbers trial No.)

Tissue

MEF

Residual Water Content

    __________________________________________________________________________    Foam Specs                                                                    Blow ratio 60:1                                                                             with filter plate I                                                                        with filter plate II                                                                      with filter plate III                  Formulation A (mesh ap. 40-100 micron)                                                                   (mesh ap. 16-40 micron)                                                                   (mesh ap. 10-16 micron)                Foam Volume: 300 ml/dm2                                                                     as FFCS      as FFCS     as FFCS                                __________________________________________________________________________    MEF (109)     180%         100%        75%                                    Blott P. (109)                                                                              115%         95%         85%                                    Tissue (109)  135%         98%         68%                                    Gauze (111)   120%         90%         79%                                    __________________________________________________________________________

Same tests, FFCS No. 10 superimposed on filter plates I, II and III.

    ______________________________________                                                   Filter   Filter   Filter                                                      Plate I  Plate II Plate III                                        ______________________________________                                        MEF (109)    82%        85%      84%                                          Blott.Paper (109)                                                                          98%        95%      100%                                         Tissue (109) 80%        85%      85%                                          Gauze (113)  90%        86%      86%                                          ______________________________________                                    

The FFCS in direct contact with the APSM determines predominantly thedewatering effect.

    ______________________________________                                                    Water content prior                                                           to dewatering                                                     ______________________________________                                        MEF         150-160%                                                          Blott.Paper 140%                                                              Tissue      160-170%                                                          Gauze       130-150%                                                          ______________________________________                                                    Water content after                                                           Strong Mangling (2 passages)                                      ______________________________________                                        MEF         140%                                                              Blott.Paper  95%                                                              Tissue      135%                                                              Gauze       105%                                                              ______________________________________                                    

EXAMPLE 7 Influence of FFCS: Dewatering Effect in % owf

    __________________________________________________________________________    APSN                                                                          Formulation A                                                                 Foam. Rate                                                                    60:1    Fibre Stock.sup.1                                                                     MEF     Blott. Pap.                                                                           Tissue  MEF                                   __________________________________________________________________________    FFCS    none                                                                              No. 10                                                                            none                                                                              No. 10                                                                            None                                                                              No. 10                                                                            None                                                                              No. 10                                                                            None                                                                              Wire-                                                                         screen                            Dewat. Eff.                                                                           190%                                                                              138%                                                                              195%                                                                              85% 115%                                                                              98% 135%                                                                              80% 24% 66%                               (Trial) (15)                                                                              (15)                                                                              (120)                                                                             (120)                                                                             (109)                                                                             (109)                                                                             (109)                                                                             (109)                                                                             *   *                                 __________________________________________________________________________     .sup.1 two layers of surgical gauze                                           *dynamic test (continuous treatment)                                     

EXAMPLE 8 Relation Air of Permeability/Dewatering Effect of FFCS (Trial33)

APSM: MEF

Formulation A

Blow Ratio 65:1

    ______________________________________                                                    Air Permeability                                                                           Dewatering                                           FFCS No.    (1/m.sup.2 /sec)                                                                           Effect (% owf)                                       ______________________________________                                        8a Filter Fabrics ZF                                                          32          1250         139%                                                 31          2100         138%                                                 46          4100         175%                                                 44          4400         185%                                                 37          5000         190%                                                 8b Nytal Filter Fabrics                                                       56           50           73%                                                 55           300          96%                                                 54           850         110%                                                 53          1900         118%                                                 52          2050         130%                                                 51          2900         185%                                                 Monofilament Filter Fabrics                                                   67           20          150%                                                 66           50          184%                                                 64           350         210%                                                 63          1100         225%                                                 Other Fabrics                                                                 10 Nylon     20           95%                                                  3 Cotton    22          120%                                                 11           188         128%                                                 13           220         150%                                                 18           280         177%                                                 14           300         195%                                                 ______________________________________                                    

EXAMPLE 9 Relation between Mesh Aperture/Open Surface/Air Permeabilityto Dewatering Effect

APSM=MEF,

Formulation A,

Blow Ratio 60:1

    __________________________________________________________________________        De-                                                                           wat.                                                                              Mesh      Thread                                                      FFCS                                                                              (% res.                                                                           Apert.                                                                            Mesh  Diam. Open Air  Water                                                                              Bubble Point                           No. water)                                                                            micron                                                                            Count/cm                                                                            (mm)  Surface                                                                            Permeab.                                                                           Permeab.                                                                           "real mesh aperture"                   __________________________________________________________________________    31  132 25  184.5 0.030 19   2100 485  --                                     32  142 26  165.7 0.035 173/4                                                                              1250 265  --                                     46  172 100  58.5 0.070 35   4100 690  --                                     44  185 80   74.5 0.054 353/4                                                                              4400 770  --                                     37  196 53  120.2 0.030 41   5050 850  --                                     56   73  5  101.0 2 × 0.045                                                                     1     50   9   --                                     55   97 10  190.0 0.042 31/2  300  80  --                                     54  117 15  204.1 0.034 91/2  840  50  --                                     53  119 25  182.0 0.030 203/4                                                                              1910 450  --                                     52  133 30  153.8 0.035 203/4                                                                              2050 470  --                                     51  184 53  72/104                                                                              2 × 0.043                                                                     21   2835 585  --                                     67      18                    12   8   395                                    66      19                    40   10  361                                    65      32                    85   25  223                                    64      57                    350  75  125                                    63      72                   1100 230   97                                    __________________________________________________________________________

EXAMPLE 10 Influence of Configuration on Dewatering Effect (AirPermeability APSM/FFCS)

To investigate the influence of the ratio of APSM to FFCS airpermeability three fabrics used as FFCS in other experiments werealternatively used as APSMs and FFCS in pairs, and dewatering trialswith foam formulation A were carried out (volume of foam: 300 ml/dm²,blow ratio 60:1).

Fabrics used

FFCS No. 18, air permeability 28 ltr/m² /sec

FFCS No. 3, air permeability 4.4 ltr/m² /sec

FFCS No. 10, air permeability 2.7 ltr/m² /sec ##EQU1## Test 10a: No. 18as APSM

No. 3 as FFCS

Test 10b:

No. 3 as FFCS

No. 18 as APSM

Test 10c:

No. 18 as APSM

No. 10 as FFCS

Test 10d:

No. 10 as APSM

No. 18 as FFCS

Test 10f:

No. 3 as FFCS

No. 10 as FFCS

Results

    ______________________________________                                               Residual Water Content % owf                                                  No. 18    No. 3       No. 10                                                    as      as      as    as    as    as                                 Ratio    APSM    FFCS    APSM  FFCS  APSM  FFCS                               ______________________________________                                        10a  6.4     54%     --    --    77%                                          10b  0.16    --        62% 76%   --                                           10c  10.0    54%     --                --      23%                            10d  0.09    --      59.7%               24% --                               10e  0.6                   --    73%   21.5% --                               10f  1.62                  71%   --    --    23.6%                            ______________________________________                                    

The results show that a ratio higher that 1 tends to give better resultsthan a configuration where the APSM has an air permeabilitysubstantially lower than that of the FFCS

EXAMPLE 11 Influence of Blow Ratio on Dewatering Effect

11a: (9)

FFCS: No. 10

APSM:MEF

Formulation A

Volume of foam constant, weight of liquid variable. Volume of foam 300ml/dm².

    ______________________________________                                        Blow                                                                          Ratio 300     150     100   75   60   50   38   30                            ______________________________________                                        Dewat.                                                                              115%    100%    90%   80%  75%  70%  68%  68%                           Effect                                                                        (a)                                                                           Dewat.                                                                               95%     85%    68%   67%  65%  63%  63%  62%                           Effect                                                                        (b)                                                                           ______________________________________                                         (a) low vacuum exposure time                                                  (b) double vacuum exposure time of (a)                                   

11b: (12)

FFCS: No. 10

APSM:MEF

Formulation A

Volume of foam varied, weight of liquid foamed constant (1 g/dm²)

    ______________________________________                                        Blow ratio                                                                              450       400    300     200  50                                    ______________________________________                                        Dewat.    98%       90%    80%     82%  73%                                   Effect                                                                        ______________________________________                                    

11c: (10)

FFCS: Mesh Apert. 40-100 micron

APSM: Tissue, Blotting Papier, Formulation A and C

Volume of foam constant, weight of foamed liquid variable

    ______________________________________                                        Blow ratio  300     75         50    30                                       ______________________________________                                        Blott. Paper                                                                  Form. B     --      100%       102%  --                                       Form. C     102%    102%       92%   --                                       Tissue                                                                        Form. B     75%     --         75%   --                                       Form. C     80%     --         --    78%                                      ______________________________________                                    

11d:

FFCS, Mesh Aperture 40 - 100

APSM: Gauze (8x)

Formulation A

Foam volume constant (200 ml), weight of foam liquid varied

    ______________________________________                                        Blow Ratio                                                                             200     165     120   60    40    20                                 ______________________________________                                        weight of                                                                              0.6 g   1.2 g   1.7 g 3.4 g 4.5 g 9 g                                liquid                                                                        Dewat.   135%    132%    136%  125%  116%  110%                               effect                                                                        ______________________________________                                    

EXAMPLE 12 Influence of Volume of Foam

12(11)

FFCS: No. 10

APSM:

Blotting Paper

MEF

Gauze

Formulation A

Blow Ratio 60:1

    ______________________________________                                        Foam      Dewatering Effect                                                   Volume    (Resid. water % owf)                                                (ml/dm.sup.2)                                                                           Blott. Paper   MEF     Gauze                                        ______________________________________                                        100       95%            80%     93%                                          200       95%            80%     90%                                          400       105%           80%     90%                                          600       --             80%     --                                           700       --             80%     --                                           ______________________________________                                    

12b: (27)

FFCS: No. 10

APSM: Gauze

Formulation A

Blow Ratio: 65:1

    ______________________________________                                        Foam Volume     Dewatering Effect                                             (ml/dm.sup.2)   (Resid. Water owf)                                            ______________________________________                                        100                 92%                                                       200                 91%                                                       300                 90%                                                       400                 89%                                                       500                 95%                                                       50       ml water   113%                                                               (not foamed)                                                         ______________________________________                                    

12c: (118)

FFCS: No. 10

APSM:

MEF

Blotting Paper

Formulation A

Blow Ratio: 60:1

    ______________________________________                                               Foam Volume (ml/cm.sup.2)                                                     100  200    400     600  700  Residual Water                                  Resid. Water (% owf)                                                                            Mangle-treated                                       ______________________________________                                        MEF       80%   80%     80%  80%  80%  110%                                   Blott. Paper                                                                           93%    94%    105%            120%                                   ______________________________________                                    

EXAMPLE 13 Influence of Surfactant Concentration (Foam Stability)

13a: (13)

FFCS: No. 10

APSM: Tissue (handkerchief)

Blow Ratio: 50:1-70:1

Formulations A and C

    ______________________________________                                        Foam                                                                          Volume   100 ml              200 ml                                           (ml/dm.sup.2)                                                                          Form. A  Form. C    Form. A                                                                              Form. C                                   ______________________________________                                        Resid.   102%     96%        102%   96%                                       Water                                                                         (% owf)                                                                       ______________________________________                                    

13b: (10)

FFCS: No. 10

APSM:

Blotting Paper

Tissue

Blow Ratio: Varied

Formulations B and C

    __________________________________________________________________________           Blow ratio                                                                    300       75        50        45-40                                           Form. B                                                                            Form. C                                                                            Form. B                                                                            Form. C                                                                            Form. B                                                                            Form. C                                                                            Form. B                                                                            Form. C                                    Resid. Water (% owf)                                                   __________________________________________________________________________    Blott. Paper                                                                         --   102% 102% 100% --   --   105% 72%                                 Tissue 72%   78% --   --   72%  --   --   78%                                 __________________________________________________________________________

13c: (123) Foam Collapse Time (with and without vacuum)

    ______________________________________                                                    Foam Collapse Time                                                              under vacuum                                                    Concentr.              min-    room pressure                                  Surfactant                                                                            g/litre   seconds  utes  seconds                                                                              minutes                               ______________________________________                                        Sandozin                                                                              15        --       15    --     >60                                   NIT     2         --       7     --     55                                    (Sandoz)                                                                              1         --       9     --     52                                            0,2       --       32    --     42                                            0,1       45       --    --     30                                    Irgapodol                                                                             1         --       5     --     15                                    FA                                                                            (. . .)                                                                       Irgapodol                                                                             1         105      --    --     40                                    FC      0.1       42       --    --     30                                    Gafac   1         --       7     --     40                                    IRA600  0,1       160      --    --     30                                    Sandopan                                                                              2         73       --    --     40                                    DTC     1         62       --    --     50                                            0.2       30       --    --     17                                            0.1       32       --    --     25                                    ______________________________________                                    

EXAMPLE 14 Influence of Initial of Water Content of FFCS on DewateringEffect on APSM

14a: (103)

FFCS: No. 10

APSM: Gauze

Formulation A

Blow Ratio: 60:1

    ______________________________________                                        Water         0%      23%     40%   55%   75%                                 Content FFCS                                                                  before Dewatering                                                             Dewat. Eff. on APSM                                                                        110%    105%    103%  102%  100%                                 (Res. Wat. owf)                                                               ______________________________________                                    

14b:

FFCS: No. 10

ASPM: MEF

Formulation A

Blow Ratio: 60:1

    ______________________________________                                        Water       0%      25%      30%   40%    50%                                 content FFCS                                                                  before Dewat.                                                                 Dewat. Eff.                                                                              108%    110%     102%  105%   106%                                 on APSM                                                                       (res. water owf)                                                              Foam Content        20%            45%                                        (% owf) on                                                                    FFCS before                                                                   Dewat.                                                                        Dewat. Eff.        100%           105%                                        on APSM                                                                       (res. wat.                                                                    cont. owf)                                                                    ______________________________________                                    

EXAMPLE 15 Influence of Swelling on Air Permeability of Water-SwellableFFCS's

    ______________________________________                                                       Air Permeability                                               ______________________________________                                        FFCS No. 3 (Cotton)                                                                            dry 80-90*  wet 25-35*                                       Cotton broad cloth                                                                             dry 760*    wet 440*                                         ______________________________________                                         *ltr/m.sup.2 /sec                                                        

EXAMPLE 16 Influence of Vacuum Exposure Time

FFCS: No. 10

APSM: MEF

Formulation A

Blow Ratio: varied

    ______________________________________                                        Blow Ratio                                                                             150          60         25                                           ______________________________________                                        Vac Exp. a      b      c    a   b    c   a    b   c                           resid.wat.                                                                             118    102    84   80  73   65  80   67  63                          % owf                                                                         ______________________________________                                         a:b:c = 1:2:4 vacuum exp. time                                           

EXAMPLE 17

(18a) Removal of Water Containing Agents

FFCS: No. 56

APSM: Cotton Broadcloth, not mercerised

Foam Blow Ratio: 60:1

Formulation A

The fabric was padded in caustic solution of mercerising strength (266 gNaOH/liter), then it was dewatered with foam (sucked through the fabric,with FFCS No. 56 between vacuum and APSM) repeatedly. Foam volume 200ml/dm², formulation A, blow ratio 65:1. No rinsing liquid was applied tothe fabric between foam dewatering treatments. The foam temperature was20° C.

Results

The water content of the highly swollen cotton fabric dropped from 104%owf to, 81.9% owf, the caustic content from 0.5288 g/dm², i.e. 52.88g/m² (=100%) to 0.1040 g/dm², i e. 10.4 g/m² (=19.7% of the originalvalue), which corresponds to a concentration of 52.3 g NaOH/liter. Inplant practice, a lowering of the caustic concentration from 266 gNaOH/liter to 56 g NaOH/lite by multiple cold and warm rinsing isconsidered satisfactory (at this concentration, a cotton fabric aftermercerising may be released from width-retaining devices with riskingsubstantial shrinkage). Five foam dewatering treatments (cold) haveachieved better caustic removal.

17b

A mercerised cotton fabric (scoured, bleached broad cloth) was padded incaustic (266 g NaOH/liter), the add-on being 101% owf.

The fabric was then treated in different ways to remove as much causticas possible with a minimum of rinsing water.

Sample 1 as dewatered one to five times with foam (formulation A, 300ml/dm² each time, no intermediate adding of water, blow ratio 65:1. FFCSNo. 56--same formulation, same weight of water).

All these treatments were carried out at room temperature.

Sample 2 was rinsed 5 times with 200 ml cold water/dm², i.e. more than30 times the weight used in foamed form.

Sample 3 was treated as Sample 2, but with 200 ml/dm² of hot water (72°C.).

17c): (124c)

Same fabric, same caustic treatment as in Example 13b. Dewatering withfoam under the same conditions as in Example 13b.

    __________________________________________________________________________                   residual caustic                                                                              total volume                                                  (% of caustic                                                                         residual                                                                              of rinsing                                                    present be-                                                                           water cont.                                                                           water used                                                    fore dewat.)                                                                          owf     (liter/kg fabric)                              __________________________________________________________________________    (a)                                                                             one foam dewaterg.                                                                         49.1%   87.9%   2.53 l/kg                                        treatments                                                                  (b)                                                                             two foam de- 29.0%   78.5%   5.30 l/kg                                        watering treatm.                                                            (c)                                                                             three foam de-                                                                             18.3%   74.8%   8.1 l/kg                                         watering treatm.                                                            (d)                                                                             one treatment with                                                                         49.0%   97.0%   2,94 l/kg                                        unfoamed water                                                                sucked through (same                                                          weight as in (a))                                                           (e)                                                                             three treatments                                                                           35.8%   103%    5.88 l/kg                                        with unfoamed water                                                           sucked through (same                                                          weight as in (c))                                                             Fabric before dewatering                                                                   100%    100%    --                                             __________________________________________________________________________

EXAMPLE 18 Dewatering of fiberstock (cotton, scoured and bleached,surgical cotton grade) (15)

FFCS: No. 10

Formulation A

Blow ratio: 60:1

Foam volume: 300 ml/dm²

    ______________________________________                                                 Residual Water Content (% owf)                                                one layer of cotton                                                                       two layers of cotton                                     ______________________________________                                        plain water                                                                              180%          275%                                                 sucked through                                                                Formulation A                                                                            165%          335%                                                 (not foamed)                                                                  sucked through                                                                Formulation A                                                                            135%          135%                                                 foamed sucked                                                                 through                                                                       ______________________________________                                    

EXAMPLE 19 Dewatering of Pile Fabric (125)

19a: Dewatering of wet terry towel fabric (cotton, 521 g/square meter,scoured, bleached and dyed).

Formulation A, foam blow ratio 60:1, 300 ml foam/dm²

FFCS No. 10: residual water content 125%

FFCS No. 56: residual water content 117.5%

19b: Dewatering of wet corduroy (cotton, 347 g/sq.meter, scoured,bleached, dyed)

Formulation B, foam blow ratio 65:1, 300 ml foam/dm²

    ______________________________________                                                    Residual water                                                                content owf                                                       ______________________________________                                        Mangle          65%                                                           FFCS No. 56   58.5%                                                           ______________________________________                                    

EXAMPLE 20 Vacuum Data, Vacuum Effects

20a: Foam Permeation Time Through Different APSM's

600 ml of foam (formulation A, blow ratio 65:1) were sucked through todifferent ASPM's. Permeation time and 6 different FFCS foam permeationtime was determined (sec).

    ______________________________________                                        FFCS                                                                          APSM    No. 37  No. 11  No. 31 No. 3 No. 46                                                                              No. 10                             ______________________________________                                        Blott. Pap.                                                                           22      32      35     48    65     95                                Tissue  23      24      23     29    28    108                                ______________________________________                                    

EXAMPLE 21 Dewatering with Wire Screen Acting as Conveyor Belt

A nonwoven (MEF) containing about 220% of water was (a) dewatered withvacuum by vacuum travelling on a wire screen (. . . mesh) across avacuum slot. To determine the influence of dewatering with foam (vsdewatering in a conventional way with vacuum) and the influence of theFFCS, the same trial was carried out (b) without foam and (c) with foamwithout an FFCS.

    ______________________________________                                                         Water Content                                                ______________________________________                                        MEF before dewatering                                                                            250%                                                       MEF vacuum treated with-                                                                         243%                                                       out foam                                                                      MEF vacuum treated with*                                                                         218%                                                       foam with FFCS                                                                MEF vacuum treated with*                                                                          70%                                                       foam on FFCS                                                                  ______________________________________                                         *Blow ratio 35:1                                                         

EXAMPLE 22 Lowering of Foaming Rate During Dewatering

APSM: Gauze

FFCS: 40-100 micron mesh aperture

22a:

Formulation A

Blow ratio 40:1 before permeation through system

Blow ratio 21:1 after permeation

Pot life of foam

before permeation: 60 minutes

after permeation: 25 minutes

Dewatering effect: 80% owf

22b:

Formulation C

Blow ratio 40:1 before permeation through system.

Blow ratio virtually zero after permeation (foam practically completelyconverted into water).

Dewatering effect: 73% owf

22c:

Formulation C

Blow ratio 65:1 before permeation,

Blow ratio practically nil after permeation

Dewatering effect 106%

22d: Same trial, but without APSM (foam sucked through FFCS only).

    ______________________________________                                        Blow ratio before                                                             permeation through                                                                            Blow ratio after                                              FFCS            permeation                                                    ______________________________________                                        86:1            77:1                                                          66:1            58:1                                                          46:1            56:1                                                          liquid          27:1                                                          ______________________________________                                    

EXAMPLE 23

A MEF nonwoven (air permeability 1200 1/m² /sec) was dewatered bypassing it in wet state (water content 180-220% owf) across two vacuumslots. The web was riding on a bronze wire mesh (air permeability 5'5001/m² /sec). Residual water content after the treatment was 65% to 70%owf within the batch of a dynamic test. These results show that even ifproperly selected, FFCS has an air permeability substantially higherthan the APSM excellent results can be obtained.

EXAMPLE 24

Comparison between water and foam sucked through APSM (with and withoutFFCS) and unfoamed water containing surfactant present in APSM producingfoam under the action of vacuum with and without FFCS--test series130--).

    ______________________________________                                        Example 24                                                                           Water                          Water                                          content                        content                                        before                  FFCS   after                                   Test No.                                                                             treatment Treatment     present                                                                              treatment                               ______________________________________                                        130.1a 210%      300 ml/dm.sup.2                                                                             no     184%                                                     sucked through                                               130.1b 212%      as 130.1a     yes    73.5%                                   130.2a 209%      10 ml/dm.sup.2 sucked                                                                       no     220%                                                     through (unfoamed                                                             formul. A)                                                   130.2b 210%      as 130.2a     yes    120%                                    130.3a 196%      10 ml/dm.sup.2 pure                                                                         no     220%                                                     water, sucked                                                                 through                                                      130.3b 205%      same as 130.3a                                                                              yes    128%                                    130.4a 190%      just vacuum   no     180%                                                     applied to wet                                                                web                                                          130.4b 209%      same as 130.4a                                                                              yes    129%                                    130.5a 210%      web dipped in no     212%                                                     formulation A,                                                                unfoamed vacuum                                                               applied                                                      130.5b 208%      same as 130.5b                                                                              yes    115%                                    --     210%      strong mangle --     118%                                                     treatment                                                    ______________________________________                                    

Remarks

(1) Tests "a" compared to tests "b" show influence of FFCS.

(2) Test 130.1b shows the superior effects of the treatment according tothe invention over the other variations.

(3) Tests 130.1a/1b compared to tests 130.2a-130.3b show the superiorityof foam over unfoamed formulations.

(4) Tests 130.4a/4b to 130.5a/5b shows that the process claimed in U.S.Pat. No. 4,062,721 (Geyer) does not produce results substantiallydifferent from those obtained with conventional vacuum extraction orremoval of water by mangling.

I claim:
 1. A process for dewatering an air permeable sheet materialcontaining water, which process comprises:applying to one side of an airpermeable sheet material foam containing an agent capable of loweringthe surface tension of the foam liquid; causing the foam to permeate theinterstices of the sheet material by application of a pressure gradientacross the sheet material; and removing the foam material and water fromthe other side of the sheet material, whereby the foam liquid causeswater in the air permeable sheet material to be substantially removedfrom the interstices of the sheet material.
 2. A process as claimed inclaim 1 wherein the pressure gradient is provided by mechanicallyforcing the foam therein.
 3. A process as claimed in claim 1 wherein thepressure gradient is established by providing pressure to the side ofthe sheet material to which he foam is applied.
 4. A process as claimedin claim 1 wherein the pressure gradient is established by theapplication of a vacuum to the side of the sheet material remote fromthat to which the foam is applied.
 5. A process as claimed in claim 1wherein the foam is in the form of an aqueous foam.
 6. A process asclaimed in claim 1 wherein the foam is in the form of a non-aqueousfoam.
 7. A process as claimed in claim 1 wherein the foam is in the formof an emulsion.
 8. A process as claimed in claim 1 wherein the agentcapable of lowering the surface tension is one which decomposes at atemperature within the range of 50° C. to 200° C. whereby the agent isremoved during any subsequent drying or heat treatment.
 9. A process asclaimed in claim 1 wherein the size of the foam cells is fairly uniform.10. A process as claimed in claim 1 wherein the maximum cell size of thefoam is not more than 1/4 the thickness of the air permeable sheetmaterial to which it is applied.
 11. A process as claimed in claim 1wherein the foaming rate of the foam applied to the sheet material iswithin the range of 300:1 to 5:1.
 12. A process as claimed in claim 11wherein the volume of foam permeating the sheet material is such thatthe foaming rate of the foam removed from the air permeable sheetmaterial after passage therethrough is 10 to 80% lower than the foamingrate of the foam originally applied.
 13. A process as claimed in claim 1wherein the operating conditions as regards foam stability, foam volume,foam rate, and foam pressure applied, are such that the foaming rate ofthe foam emerging from the air permeable sheet is less than 50% of thefoaming rate of the foam applied to the air permeable sheet material.14. A process as claimed in claim 1 wherein a foam flow constrainingsubstrate is in juxtaposition with the air permeable sheet material tosupport the same during the foam treatment.
 15. A process as claimed inclaim 14 wherein the foam flow constraining substrate is juxtaposed theair permeable sheet material on the side remote from that to which thefoam is applied.
 16. A process as claimed in claim 14 wherein the foamflow constraining substrate is juxtaposed the air permeable sheetmaterial on the side thereof to which the foam is applied.
 17. A processas claimed in claim 14 wherein the foam flow constraining substrate isarranged to move with the air permeable sheet material.
 18. A process asclaimed in claim 14 wherein the foam flow constraining substrate is asheet material having porous characteristics ensuring a substantiallyuniform permeation of air, liquid and foam through the intersticesthereof, said substrate having an air permeability at least equal to theair permeable sheet material to be treated.
 19. A process as claimed inclaim 14 wherein the dimension of pores or interstices of the foam flowconstraining substrate is not more than 50 microns.
 20. A process asclaimed in claim 14 wherein the foam flow constraining substrate is awoven fabric, a nonwoven web, or a mesh.
 21. A process as claimed inclaim 14 wherein the foam flow constraining substrate is a woven fabrichaving an air permeability of not more than 250 liters per square meterper second or a non-woven structure or mesh having an air permeabilityof not more than 2000 liters per square meter per second.
 22. A processas claimed in claim 14 wherein said substrate is maintained in closecontact with said sheet material throughout the treatment with the foam.23. A process as claimed in claim 14 wherein the foam is caused topermeate the interstices of the sheet material by means of a pressuregradient, said pressure gradient being generated by means of a vacuumapplied on the side of the air permeable sheet material remote from theside on which the foam is applied, said vacuum being applied by passingthe air permeable material across at least one vacuum slot, each vacuumslot being defined by an open tube pipe or duct connected to a vacuumproducing pump.
 24. A process as claimed in claim 23 wherein these aremultiple vacuum slots are arranged in a plane, a curve, or within arotating drum.
 25. A process as claimed in claim 24 wherein thesubstrate is caused to travel at an angle of not more than 60° to thehorizontal plane when traversing said vacuum slot.
 26. A process asclaimed in claim 1 wherein the foam includes at least one agent for theremoval of deleterious matter from said air permeable sheet material.27. A process as claimed in claim 1 wherein the said air permeable sheetmaterial is dry when the foam is first applied.
 28. A process as claimedin claim 1 wherein the air permeable sheet material is wet when the foamis first applied.
 29. A process as claimed in claim 26 wherein the foamadditionally contains compounds capable of neutralizing, emulsifying,and/or dispersing delterious matter or agents present in said sheetmaterial.
 30. A process as claimed in claim 1 wherein a furtherapplication of foam containing an agent capable of lowering the surfacetension is applied to the air permeable sheet material, said foam beingcaused to permeate the interstices of the sheet material and thereafterremoving the foam and/or constituents of the foam from the sheetmaterial.
 31. A process as claimed in claim 1 wherein the foam liquidapplied to the air permeable sheet material contains agents to beinteracted with or deposited into said air permeable sheet material. 32.A process as claimed in claim 29 wherein the amount of water present inthe air permeable sheet material is within the ±25% of the minimum foamtransit water content of the air permeable sheet material when the foamis applied to it.
 33. A process as claimed in claim 1 further comprisingforming the foam containing the agent.